The present invention relates to a process and device for inerting an aircraft fuel tank.
The invention contemplates all kinds of aircraft, in particular airplanes and helicopters. It is applicable to any type of flight, whether civil or military.
During the flight of an aircraft, there exists the continuous risk of ignition of the fuel vapors. The occurrence of this ignition can be reduced, by injecting into the reservoir an inert gas, permitting lowering the percentage of oxygen that is present there, below a predetermined value. Once this latter is reached, the mixture present in the reservoir is substantially no longer explosive. Such a process is for example described in WO-A-99/34 106.
It is also known to produce, on board the aircraft, air enriched in nitrogen or, in other words, impoverished in oxygen. Such a production is carried out by means of an apparatus relying on permeable membranes, or else on molecular sieves. This apparatus, which communicates with a source of compressed air internal to the aircraft, permits supplying, on the one hand, a fraction of air enriched in oxygen and, on the other hand, a fraction of output air enriched in an inert gas, particularly nitrogen. This latter fraction is injected into the reservoir, so as to render it inert.
This known solution however has certain drawbacks. Thus, to have complete protection, the fuel tank must be rendered inert during all of the flight. This requires providing apparatus for supplying air enriched in inert gas, whose weight and size are great, which correspondingly renders the aircraft heavier. Moreover, such apparatus is of high cost. Finally, it requires a significant consumption of air under pressure, which gives rise to a drop in the overall power of the aircraft, to the extent that this consumption limits the propulsion of it.
So as to overcome these various drawbacks, the invention proposes to practice a process which, whilst ensuring reliably rendering inert the vapor contained in the fuel tank, uses mechanical elements whose size and weight are reduced relative to the prior art, and which involves a lesser supply of compressed air than in the prior art.
To this end, it has for its object a process for inerting an aircraft fuel tank, in which there is produced air enriched in inert gas, from a source of compressed air internal to said aircraft, and said enriched air is admitted into the tank containing said fuel, in which, during at least one cruising phase of a flight of the aircraft, there is produced a first fraction of air having a first content of inert gas, and this first fraction is admitted into the reservoir at a first flow rate, and, during a descent phase of said flight, there is produced a second fraction of enriched air, having a second content of inert gas, substantially less than that of the first content, and there is admitted into the fuel tank the second fraction and ambient air, at a second flow rate substantially greater than said first flow rate.
The invention permits achieving the previously mentioned objectives.
Thus, if it is desired to give complete protection to the aircraft, it is necessary to render inert the gas tank during the entire flight, which is to say the phases of takeoff and ascent, then cruising, and finally descent and landing.
However, the most troublesome phase, in terms of the quantity of gas to be generated, is during descent. Thus, this phase is accompanied by an increase of the atmospheric pressure and, because of this, a corresponding increase of the mass of gas contained in the volume occupied by the gaseous phase, in the fuel tank. There must thus be injected a supplemental quantity of inert gas, so as to compensate for this increase. The descent thus constitutes the determining phase, as to the dimensioning of the apparatus for generating air enriched in inert gas.
The invention seeks to provide, during the cruising phase at high altitude, air particularly enriched in inert gas, with a content greater than the theoretical content at which the risk of explosion is absent.
It should be noted that this measure however involves no significant supplemental cost. Thus, during this cruising phase, the quantity of enriched air to be produced is particularly low, because it corresponds substantially only to the replacement of consumed fuel.
According to the invention, at the end of the cruising phase, the fuel tanks of the aircraft contain air whose content of inert gas is high or, in other words, which is particularly impoverished in oxygen. Under these conditions, it is thus acceptable to inject, during the descent, air whose content of inert gas is below the theoretically safe content, mentioned above.
This air which is less enriched in inert gas, which can be obtained by dilution with ambient air, will thus be mixed with the particularly enriched air, previously injected into the fuel tank during the cruising phase. It will be seen that such mixing operation will lead to the obtention of air whose content in inert gas will be near the mentioned safe content.
By way of example, it is thus possible to provide, during the cruising phase, air containing about 2% of oxygen and, during the descent phase, air containing 12% oxygen. During descent, it is also acceptable to admit, within the fuel tank, a certain quantity of ambient air.
The fact of injecting, during descent, air whose content in inert gas is relatively low, is particularly advantageous from an economic standpoint. Thus, as this descent phase corresponds to maximum requirement for inert gas, it is possible to provide for the apparatus for generating enriched air, dimensions which are substantially reduced relative to the prior art. Thus, in the known technique, the inert gas is produced at a substantially constant content, during all of the flight.
This reduction in the size of equipment thus permits a reduction of the mass, as well as the cost of manufacture. Thus, it permits decreasing the air consumption, provided in the aircraft circuit, which decreases the performance of this latter only to a small extent.
According to other characteristics of the invention:
the ratio between the second flow rate and the first flow rate is comprised between 2.5 and 5;
the second content of inert gas is comprised between 86 and 95%.
The invention also has for its object a device for inerting an aircraft fuel tank, comprising: means for producing air enriched in inert gas, which can be placed in communication with a source of compressed air, internal to the aircraft, these production means being moreover placed in communication with the tank that receives the fuel, means permitting regulating the content of inert gas flowing from the outlet of the production means, and means for bringing outside air into the internal volume of the fuel tank.